The present invention relates to optical systems which have a focal plane and in particular to structures for detecting when an image is properly focussed at the focal plane.
Thus, the present invention may be used for detecting the focal adjustment of an optical system of a photographic camera for the purpose of focussing an image of an object which is to be photographed.
It is known to be possible to detect when an optical system of the above type is properly adjusted so as to focus an image at the desired focal plane. For this purpose advantage is taken of a phenomenon known as the dip effect which occurs during variation of the contrast of an optical image when a photoconductive element such as a cadmium sulfide (Cds) element is used to receive light for the purpose of detecting the focal adjustment.
It is well known that this dip effect depends upon three factors, namely, the space frequency, the contrast, and the light intensity of an object such as an object which is to be photographed. As a result, the particular configuration of a dip curve will vary considerably depending upon these factors. When it is required to electrically process such wide variations in a dip curve, the electrical circuitry provided for this purpose must have an extremely wide dynamic range. As a result, problems of variation in the sensitivity of detection of the focal adjustment are encountered.
When utilizing an automatic focalizing device for an optical system of a photographic camera or the like, wherein it is essential to be capable of receiving light which has a relatively wide range of intensity, the desired operation is adversely influenced particularly by the third of the above factors, namely, the variation in brightness or light intensity at the object which is to be photographed.
The influence of the variation in the light intensity upon the dip effect falls into two categories, namely one according to the characteristics peculiar to the particular photoconductive element itself and another according to the particular features of the detecting circuit.